Cationic electrodeposition coating compositions are easy to apply, and form a highly corrosion-resistant coating film. They are widely used as an undercoat for conductive metal products, such as automotive bodies, which require such features.
In an effort to enhance collision safety, automotive bodies have been strengthened, and reinforcing materials have been further added to spot-welded members. This, however, has greatly increased the number of substrates to be coated that structurally include complicated depressed portions, gap portions, and like portions. Such structures lower the current density (mA/cm2) during electrodeposition coating, making it difficult to deposit coating films. Thus, depressed portions, gap portions, and like portions may not be coated, and the corrosion resistance of the portions may be greatly lowered, particularly in a severe environment where snow-melting salt is spread.
To secure the film thickness (μm) of the depressed portions, gap portions, and like portions (i.e., to improve the “throwing power”; hereinafter may be referred to as “throwing power”), increased voltages are applied for coating; however, this may deteriorate the finished appearance, or increase the amount of coating composition for use because of thickened coating films (μm) formed on exterior panels (i.e., substrates).
As an effective means for securing a film thickness of the depressed portions, gap portions, and like portions, an electrophoretic coating composition comprising carboxy-containing polyesters (A) having an acid value of 30 to 150 mg KOH/g and a hydroxy value of 20 to 150 mg KOH/g is disclosed (Patent Document 1). However, Patent Document 1 is directed to an anionic electrodeposition coating composition, and the corrosion resistance of obtained coating films is insufficient.
A prior art document similar to the present application discloses a cationic electrodeposition coating composition comprising (A) an epoxy-type cationic electrodepositable resin having a specific surface tension and (B) a polyester resin having a specific surface tension (Patent Document 2). However, Patent Document 2 is directed to a multilayer coating film in which a resin layer having excellent corrosion resistance is distributed in the lower-layer portion, and a resin layer having excellent weatherability is distributed in the upper-layer portion; therefore, finished appearance and throwing power of this electrodeposition coating composition is insufficient.
An electrodeposition coating composition that contains an epoxy amine adduct resin and a hydroxy-terminated polyester resin with a molecular weight of 200 to 1,000 is disclosed (Patent Document 3). However, in Patent Document 3, the hydroxy-terminated polyester resin is simply used as an additive, and the corrosion resistance is insufficient. In addition, the coating composition stability and finished appearance are lowered.
An electrodeposition coating composition that contains a polyester compound obtained by a reaction of aliphatic glycol, aliphatic dicarboxylic acid, and aliphatic monohydric alcohol is disclosed (Patent Document 4). However, the electrodeposition coating composition disclosed in Patent Document 4 may have reduced stability and insufficient corrosion resistance.
A method for producing an electrodeposition coating composition by mixing an aqueous dispersion (emulsion) of (a) an anionic polyester resin and an aqueous dispersion (emulsion) of (b) a cationic epoxy resin is disclosed (Patent Document 5). However, when the coating line is overloaded, the electrodeposition coating composition produced by mixing aqueous dispersions of different resin compositions may become unstable, blocking the UF filter, and thereby resulting in degraded finished appearance.
A cationic electrodeposition coating composition containing an aqueous polyester resin that has a number average molecular weight of 1,000 to 10,000, an acid value of 20 to 80, and a hydroxy value of 50 to 200 is disclosed (Patent Document 6). However, when a polyester resin having an acid value of 20 to 80 is used for a cationic electrodeposition coating composition, the stability of the coating composition may be reduced. This may lead to frequent blockage of the UF filter, and degrade the finished appearance.
A cationic electrodeposition coating composition is disclosed that contains a cation-modified epoxy resin, an anionic polyester resin, a fluorine resin, and a blocked polyisocyanate, wherein the solubility parameter δb of the cationic epoxy resin and the solubility parameter δa of the anionic polyester resin satisfy the relationship (δb−δa)≧1.0 (Patent Document 7). However, the compatibility between the cationic epoxy resin and the anionic polyester resin is poor, and the finished appearance may be degraded.